1. Field of the Invention
The present invention relates to a motor, and specifically relates to a motor having a rotor of optimized shape.
2. Description of Related Art
It is generally known that the fundamental frequency component (hereinafter referred to as “fundamental component”) of a cogging torque per rotation of a rotor, which can be reduced by optimizing the shape of the rotor, appears at a frequency of the least common multiple of the number of poles of the rotor and the number of teeth of a stator.
To reduce the fundamental component of a cogging torque, rotors have been conventionally optimized in shape in various ways. FIG. 1 shows a variation in the waveform (fundamental component) of a cogging torque, when a rotor is continuously varied in shape. Assuming that, when the rotor is gradually varied in shape from (1) to (5), the cogging torque has the lowest amplitude in the shape of (3). In this case, the shape of (3) is used as an optimal shape.
Conventional shape optimization aims at reducing a fundamental component, but cannot completely eliminate the fundamental component. To eliminate the remaining fundamental component, in general, the shape of a rotor, the shape of teeth of a stator, the phase relationship between the rotor and the teeth, or the like is varied in the direction of a motor axis. As the most general method of them all, a skew structure in which the polar phase of a rotor is varied in an axial direction is known (for example, Japanese Unexamined Patent Publication (Kokai) No. 2014-150626, hereinafter referred to as “patent document 1”).
FIG. 2 is a schematic perspective view of a rotor having a two-layer skew structure in a conventional motor according to the patent document 1. A rotor 1002 has rotor core blocks 1041a and 1041b in two layers in an axial direction. In each of the rotor core blocks 1041a and 1041b, permanent magnets 1050 are embedded to form a plurality of magnetic poles. The rotor 1002 has a layered skew structure in which the rotor core blocks 1041a and 1041b are integrated into one unit in a state of being skewed relative to each other in a circumferential direction.
FIG. 3 is an enlarged view of an essential portion of a rotor core 1040 of FIG. 2 viewed from the axial direction in the conventional motor according to the patent document 1. A skew angle is established such that in a rotor core block 1041 including the rotor core blocks 1041a and 1041a of two layers, at least part of flux barriers 1060a and 1060b between adjacent magnetic poles overlap in space on both sides of the magnets 1050 fitted into magnet fitting recesses 1042. At least part of the overlapping flux barriers 1060a and 1060b are aligned across the layers in the axial direction. The overlapping flux barriers 1060a and 1060b have an approximately oval shape in cross section and approximately the same outer peripheral shape. Approximately aligning the flux barriers 1060a and 1060b across the layers intercepts a short circuit flux flowing through the layers in the axial direction.
In the conventional art of FIG. 2, the rotor is formed into a plurality of layers, and the phase of the rotor is skewed relative to each layer to skew the phase relationship between the rotor and a stator. Thereby, a cogging torque occurs in different phases and cancels out owing to superposition. Besides the above, a method of skewing a stator phase, a method of varying the shape of a rotor among layers, a method of varying the shape of teeth of a stator among layers, or the like may be used.
According to the conventional art, since the fundamental component of a cogging torque is not sufficiently reduced, rotors sometimes have a skew structure (continuous skew structure or skew structure among a plurality of layers). However, the skew structure may cause a reduction in the output of motors. Also, the skew structure increases the number of components and man-hours, thus increasing costs.